Rider plugin & optional fixes - #4

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-dotnet@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

The type 'T' cannot be used as type parameter 'T' in the generic type or method 'Codec<T>'. Nullability of type argument 'T' doesn't match 'notnull' constraint.